Nervous system

What is the basic order of the nervous system?

receptor —> sensory neurone —> synapse —> relay neurone —> synapse —> motor neurone —> effector (muscle glands)

What components are in the spinal cord?

synapse and relay neurone be between the sensory neurone and motor neurone

What side does the sensory neurone and motor neurone come out of the spinal cord?

same side (as the impulse reacting is on the same side of the body, e.g. otherwise, if left hand touches something hot, right arm would raise which is wrong, so impulse runs on the same side of the spinal cord to make the correct effect)

myelinated sensory neurone

contains receptors, dendrons (axons) and cell body and synapses. The direction of impulse is from receptors to the synapse

Myelinated motor neurone

contains the cell body, myelin sheath, axon neuromuscular junction and muscle fibre. The direction of the impulse is from the cell body to the muscle fibre

Key features and advantages of reflexes 

• do not have to be learnt/ automatic/ involuntary

• rapid - only 3 neurones involved 

• protect against damage to body tissues

• prevents injury/named injury 

• help escape from predators

• finding food/mates/suitable conditions 

• enable homeostatic control

Resting membrane potential 

1. NA+/K+ pump actively transports using ATP 3NA+ OUT of the neurone and 2K+ IN

2. SO

• a higher concentration of NA+ OUTSIDE 

• a higher concentration of K+ INSIDE

3. Differential permeability of the membrane 

• More permeability to K+ than NA+ (as K+ protein channel gates are open and Na+ channel gates are closed)

4. More K+ ions diffuse out than Na+ diffuse in (one K+ ion is able to diffuse in but one NA+ ion can’t diffuse out)

5. The membrane has a potential of -70 mV, this is the resting potential 

The action potential

1. Resting potential (-70mV). High concentration of Na+ ions outside. High concentration of K+ inside. The membrane is polarised 

2. Some Na+ channels in the membrane open. (e.g. stimulus). Diffusion Na+ move in by facilitated diffusion. Membrane starts to become less negative. start to depolarise the membrane

3. If enough Na+ ions diffuse in to reach threshold potential (-55mV) voltage gated sodium ion channels open, leading to a rapid influx of Na+ (diffusion in of Na+ ions)

4. The membrane becomes depolarised (more positive inside than outside) reading a membrane potential +40mV. This is the action potential (+inside and - outside).

5. Na+ channels close. K+ channels open.

6. K+ ions diffuse out - so membrane repolarises 

7. The membrane becomes hyperpolarised

8. The Na+/K+ pump restores resting potential 

Pacinian corpuscle 

detects pressure

features of a Pacinian Corpuscle

What part of the Pacinian corpuscle is this

Sensory neurone ending

What part of the Pacinian corpuscle is this

Sensory neurone ending

What part of the Pacinian corpuscle is this

lamellae (layers of connective tissue)

What part of the Pacinian corpuscle is this

capsule

What happens if there’s pressure detected from the Pacinian Corpuscle?

1. Stretch mediated sodium ion channels in the membrane

2. Increased pressure:

• deforms the lamella and the membrane

• deforms the sodium ion channels

3. Na+ ion channels open

4. Na+ diffuse in

5. Depolarisation of the membrane leading to a generator potential

6. If a generator potential reaches threshold potential, an action potential will be produced

Low pressure exerted

Not enough Na+ channels open, generator potential doesn’t reach threshold so no action potential

• deforms lamellae and membrane

• some stretch mediated channels open

• Na+ diffuse in (depolarisation) generator potential

High pressure exerted

more Na+ channels open, more depolarisation, generator potential reaches threshold —> action potential

‘All or nothing’ reponse

• an action potential is only generated when threshold is reached

• If threshold is not reached, there is no action potential

• Action potential is fixed in size - once threshold is reached it will always be +40mv

• The strength of the stimulus can be determined by:

‘ frequency of action potentials- the stronger the stimulus, the greater the frequency of action potentials.

‘ number of neurones affected

Refractory period 

• When the sodium channels are closing, no more sodium ions can enter 

• The neurone cannot be stimulated (at that point of the membrane) and an action potential cannot be generated 

• The refractory period ensures that:

‘ a nerve impulse travels in one direction along the neurone, 

‘ impulses cannot merge and become discrete events

Transmission of Action Potential

Factors that affect speed of conduction

• myelination of neurone

• increased diameter of neurone 

• increasing temperature 

What occurs in a relaxed muscle

tropomyosin blocks the myosin binding sites on the actin filament 

(don’t need to know about troponin)

Process of muscle contraction